We put a backup ring between the O ring and the threads on all of our valves. On many of the valves this means the backup ring is on the pressure side. Our cavity design creates a metal-to-metal seal at the locating shoulder and therefore we do not need to back up the O ring seal…….there is no place for the seal to extrude. The "backup" ring is there for a mechanical reason.When there is an O ring immediately above a thread there is a strong possibility that the thread will screw through the O ring when the joint is broken and the threaded piece is removed. This shreds the O ring. The O ring was intact until the fitting or cartridge was removed. The problem is that you now have to replace the O ring. This is most likely to occur when the joint has been assembled for some time or when the manifold is made out of steel or iron. Sun assembles a hard elastomer ring (backup ring) between the O ring and the thread. The ring is there to pull the O ring out of the bore when the cartridge is removed. At times the thread screws through the ring, damaging it. This does not impair its ability to protect the O ring a second time. If the ring is ruined, it can be discarded and the cartridge reinstalled. However, the next time the cartridge is removed the O ring may be destroyed. By the way. Sun strives to use standard seals on the outside of their valves. We explain our part numbering system and how it applies to the seal kits in our service bulletin. You should be able to use commercially available seals to service valves in the field.